Mitochondrial encephalomyopathies--fifty years on: the Robert Wartenberg Lecture

Deoxyguanosine kinase deficiency: natural history and liver transplant outcome

Autosomal recessive pathogenetic variants in the DGUOK gene cause deficiency of deoxyguanosine kinase activity and mitochondrial deoxynucleotides pool imbalance, consequently, leading to quantitative and/or qualitative impairment of mitochondrial DNA synthesis. Typically, patients present early-onset liver failure with or without neurological involvement and a clinical course rapidly progressing to death. This is an international multicentre study aiming to provide a retrospective natural history of deoxyguanosine kinase deficient patients. A systematic literature review from January 2001 to June 2023 was conducted. Physicians of research centres or clinicians all around the world caring for previously reported patients were contacted to provide followup information or additional clinical, biochemical, histological/histochemical, and molecular genetics data for unreported cases with a confirmed molecular diagnosis of deoxyguanosine kinase deficiency. A cohort of 202 genetically confirmed patients, 36 unreported, and 166 from a systematic literature review, were analyzed. Patients had a neonatal onset (≤ 1 month) in 55.7% of cases, infantile (>1 month and ≤ 1 year) in 32.3%, pediatric (>1 year and ≤18 years) in 2.5% and adult (>18 years) in 9.5%. Kaplan-Meier analysis showed statistically different survival rates (P < 0.0001) among the four age groups with the highest mortality for neonatal onset. Based on the clinical phenotype, we defined four different clinical subtypes: hepatocerebral (58.8%), isolated hepatopathy (21.9%), hepatomyoencephalopathy (9.6%), and isolated myopathy (9.6%). Muscle involvement was predominant in adult-onset cases whereas liver dysfunction causes morbidity and mortality in early-onset patients with a median survival of less than 1 year. No genotype-phenotype correlation was identified. Liver transplant significantly modified the survival rate in 26 treated patients when compared with untreated. Only six patients had additional mild neurological signs after liver transplant. In conclusion, deoxyguanosine kinase deficiency is a disease spectrum with a prevalent liver and brain tissue specificity in neonatal and infantile-onset patients and muscle tissue specificity in adult-onset cases. Our study provides clinical, molecular genetics and biochemical data for early diagnosis, clinical trial planning and immediate intervention with liver transplant and/or nucleoside supplementation.

Mitochondrial Biomarkers in the Omics Era: A Clinical-Pathophysiological Perspective

Mitochondrial diseases (MDs) affect 4300 individuals, with different ages of presentation and manifestation in any organ. How defects in mitochondria can cause such a diverse range of human diseases remains poorly understood. In recent years, several published research articles regarding the metabolic and protein profiles of these neurogenetic disorders have helped shed light on the pathogenetic mechanisms. By investigating different pathways in MDs, often with the aim of identifying disease biomarkers, it is possible to identify molecular processes underlying the disease. In this perspective, omics technologies such as proteomics and metabolomics considered in this review, can support unresolved mitochondrial questions, helping to improve outcomes for patients.

Psychometric performance of the Primary Mitochondrial Myopathy Symptom Assessment (PMMSA) in a randomized, double-blind, placebo-controlled crossover study in subjects with mitochondrial disease

BACKGROUND

The Primary Mitochondrial Myopathy Symptom Assessment (PMMSA) is a 10-item patient-reported outcome (PRO) measure designed to assess the severity of mitochondrial disease symptoms. Analyses of data from a clinical trial with PMM patients were conducted to evaluate the psychometric properties of the PMMSA and to provide score interpretation guidelines for the measure.

METHODS

The PMMSA was completed as a daily diary for approximately 14 weeks by individuals in a Phase 2 randomized, placebo-controlled crossover trial evaluating the safety, tolerability, and efficacy of subcutaneous injections of elamipretide in patents with mitochondrial disease. In addition to the PMMSA, performance-based assessments, clinician ratings, and other PRO measures were also completed. Descriptive statistics, psychometric analyses, and score interpretation guidelines were evaluated for the PMMSA.

RESULTS

Participants (N = 30) had a mean age of 45.3 years, with the majority of the sample being female (n = 25, 83.3%) and non-Hispanic white (n = 29, 96.6%). The 10 PMMSA items assessing a diverse symptomology were not found to form a single underlying construct. However, four items assessing tiredness and muscle weakness were grouped into a "general fatigue" domain score. The PMMSA Fatigue 4 summary score (4FS) demonstrated stable test-retest scores, internal consistency, correlations with the scores produced by reference measures, and the ability to differentiate between different global health levels. Changes on the PMMSA 4FS were also related to change scores produced by the reference measures. PMMSA severity scores were higher for the symptom rated as "most bothersome" by each subject relative to the remaining nine PMMSA items (most bothersome symptom mean = 2.88 vs. 2.18 for other items). Distribution- and anchor-based evaluations suggested that reduction in weekly scores between 0.79 and 2.14 (scale range: 4-16) may represent a meaningful change on the PMMSA 4FS and reduction in weekly scores between 0.03 and 0.61 may represent a responder for each of the remaining six non-fatigue items, scored independently.

CONCLUSIONS

Upon evaluation of its psychometric properties, the PMMSA, specifically the 4FS domain, demonstrated strong reliability and construct-related validity. The PMMSA can be used to evaluate treatment benefit in clinical trials with individuals with PMM. Trial registration ClinicalTrials.gov identifier, NCT02805790; registered June 20, 2016; https://clinicaltrials.gov/ct2/show/NCT02805790 .

Two Sisters with Kallmann Syndrome, Gonadal Dysgenesis, and Multiple Neuromuscular and Endocrine Disorders: Report of Two Cases with Description of an Unusual Association

Kallmann syndrome (KS) is an uncommon genetic disorder characterized by isolated congenital hypogonadotropic hypogonadism (CHH) and anosmia/hyposmia. KS originates from abnormal embryonic migration of olfactory axons and gonadotropin-releasing hormone (GnRH)-synthesizing neurons. It can be challenging to diagnose due to its heterogeneous clinical presentation and genes implied. Herein, we report a rare phenotype of KS in two sisters accompanied by a variety of nonreproductive disorders such as hypoparathyroidism, hypercortisolism, atrophy of the cerebellum, intellectual disability, and remarkably, ovarian dysgenesis. Additionally, both subjects present muscle weakness, exercise intolerance, marked hypotonia and seizures, being suspected, although not fully confirmed, mitochondrial encephalomyopathy. These cases illustrate the heterogeneous clinical presentation and the diagnostic difficulties often found in patients suffering from this condition. These clinical features have never been described before as associated with KS; therefore, we decided to report this novel KS phenotype.

Clinical Characteristics of Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-Like Episodes

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, a maternally inherited mitochondrial disorder, is characterized by its genetic, biochemical and clinical complexity. The most common mutation associated with MELAS syndrome is the mtDNA A3243G mutation in the MT-TL1 gene encoding the mitochondrial tRNA-leu(UUR), which results in impaired mitochondrial translation and protein synthesis involving the mitochondrial electron transport chain complex subunits, leading to impaired mitochondrial energy production. Angiopathy, either alone or in combination with nitric oxide (NO) deficiency, further contributes to multi-organ involvement in MELAS syndrome. Management for MELAS syndrome is amostly symptomatic multidisciplinary approach. In this article, we review the clinical presentations, pathogenic mechanisms and options for management of MELAS syndrome.

Mitochondrial DNA A3243G variant-associated retinopathy: a meta-analysis of the clinical course of visual acuity and correlation with systemic manifestations

PURPOSE

The mitochondrial DNA A3243G (m.3243A>G) variant causes a wide spectrum of phenotypes, with pigmentary retinopathy as the most common ocular finding. We undertook this meta-analysis to investigate the clinical course of visual acuity (VA) in patients with m.3243A>G variant and provide key clinical correlations with systemic manifestations.

METHODS

A PubMed literature search was performed and studies were selected after satisfying pre-set inclusion criteria. Demographic and clinical data, including retinal findings and systemic manifestations were recorded. Cross-sectional and linear regression analyses were used to investigate the relationship between VA and age, as well as between the age at diagnosis of retinopathy and the mean ages at diagnosis of sensorineural hearing loss or diabetes. The age and prevalence of systemic manifestations among patients with and without retinopathy were studied using t-tests and Mann-Whitney U-tests (performed on binarized data). Likelihood ratios were computed.

RESULTS

The mean VA (average of both eyes) of 90 patients (72.2% female; 65/90) were collected from 18 studies published between 1990 and 2018. The baseline mean age was 45.2 years (range 17 to 92). The mean logMAR VA was 0.10 (- 0.12 to 1.39). There was a statistically significant linear correlation between the logMAR VA and age (p = .008). The VA of patients less than or equal to 50 years of age was significantly better than that of patients older than 50 years (0.06 vs.0.18 logMAR, p = .002). 67 patients (74.4%) showed a characteristic pigmentary retinopathy with a mean age at diagnosis of 47.9 years (17 to 92) and VA of 0.14 logMAR (- 0.12 to 1.24). Age at diagnosis of retinopathy was linearly correlated with age at diagnosis of hearing loss or diabetes (p < .001). Patients with retinopathy were more likely to have hearing loss (83.6% vs. 56.5%, p = .03) or diabetes (56.7% vs. 17.4%, p = .001) than those without retinopathy. Those with both hearing loss and diabetes had an earlier onset of retinopathy than those without (46.4 vs. 60.4 years, p = .01). Patients without both hearing loss and diabetes were 5.3-fold less likely to develop a retinopathy.

CONCLUSIONS

Patients with m.3243A>G variant pigmentary retinopathy maintain highly functional VA until around the fifth decade of life, after which significant visual decline ensues. Patients without hearing loss and diabetes have a lower likelihood of exhibiting a retinopathy, which tends to appear about one decade after hearing loss and diabetes are diagnosed.

Mitochondrial DNA A3243G variant-associated retinopathy: Current perspectives and clinical implications

Cellular function and survival are critically dependent on the proper functionality of the mitochondrion. Neurodegenerative cellular processes including cellular adenosine triphosphate production, intermediary metabolism control, and apoptosis regulation are all mitochondrially mediated. The A to G transition at position 3243 in the mitochondrial MTTL1 gene that encodes for the leucine transfer RNA (m.3243A>G) causes a variety of diseases, including maternally inherited loss of hearing and diabetes syndrome (MIDD), mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes syndrome (MELAS). Ophthalmological findings-including posterior sub-capsular cataract, ptosis, external ophthalmoplegia, and pigmentary retinopathy- have all been associated with the m.3243A>G variant. Pigmentary retinopathy is, however, the most common ocular finding, occurring in 38% to 86% of cases. To date, little is known about the pathogenesis, natural history, and heteroplasmic and phenotypic correlations of m.3243A>G-associated pigmentary retinopathy. We summarize the current understanding of mitochondrial genetics and pathogenesis of some associated diseases. We then review the pathophysiology, histology, clinical features, treatment, and important ocular and systemic phenotypic manifestations of m.3243A>G variant associated retinopathy. Mitochondrial diseases require a multidisciplinary team approach to ensure effective treatment, regular follow-up, and accurate genetic counseling.

PET Imaging for Oxidative Stress in Neurodegenerative Disorders Associated with Mitochondrial Dysfunction

Oxidative stress based on mitochondrial dysfunction is assumed to be the principal molecular mechanism for the pathogenesis of many neurodegenerative disorders. However, the effects of oxidative stress on the neurodegeneration process in living patients remain to be elucidated. Molecular imaging with positron emission tomography (PET) can directly evaluate subtle biological changes, including the redox status. The present review focuses on recent advances in PET imaging for oxidative stress, in particular the use of the Cu-ATSM radioligand, in neurodegenerative disorders associated with mitochondrial dysfunction. Since reactive oxygen species are mostly generated by leakage of excess electrons from an over-reductive state due to mitochondrial respiratory chain impairment, PET with ⁶²Cu-ATSM, the accumulation of which depends on an over-reductive state, is able to image oxidative stress. ⁶²Cu-ATSM PET studies demonstrated enhanced oxidative stress in the disease-related brain regions of patients with mitochondrial disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Furthermore, the magnitude of oxidative stress increased with disease severity, indicating that oxidative stress based on mitochondrial dysfunction contributes to promoting neurodegeneration in these diseases. Oxidative stress imaging has improved our insights into the pathological mechanisms of neurodegenerative disorders, and is a promising tool for monitoring further antioxidant therapies.

Incidence and predictors of total mortality in 267 adults presenting with mitochondrial diseases

Assessing long-term mortality and identifying predictors of death in adults with mitochondrial diseases. We retrospectively included adult patients with genetically proven mitochondrial diseases referred to our centre between January 2000 and June 2016, and collected information relative to their genetic testing, clinical assessments, and vital status. We performed single and multiple variable analyses in search of predictors of total mortality, and calculated hazard ratios (HR) and 95% confidence intervals (CI). We included 267 patients (women 59%; median age 43.3 [31.3-54.2] years), including 111 with mitochondrial DNA (mtDNA) single large-scale deletions, 65 with m.3243A>G, 24 with m.8344A>G, 32 with other mtDNA point mutations, and 36 patients with nuclear genes mutations. Over a median follow-up of 8.9 years (0.3 to 18.7), 61 patients (22.8%) died, at a median age of 50.7 (37.9-51.9) years. Primary cause of death was cardiovascular disease in 16 patients (26.2%), respiratory in 11 (18.0%), and gastrointestinal in 5 (8.1%). By multiple variable analysis, diabetes (HR 2.75; 95% CI 1.46-5.18), intraventricular cardiac conduction defects (HR 3.38; 95% CI 1.71-6.76) and focal brain involvement (HR 2.39; 95% CI 1.25-4.57) were independent predictors of death. Adult patients with mitochondrial diseases present high morbidity that can be independently predicted by the presence of diabetes, intraventricular cardiac conduction defects, and focal brain involvement.

Mitochondrial genetic medicine

Inherited mitochondrial DNA (mtDNA) diseases were discovered 30 years ago, and their characterization has provided a new perspective on the etiology of the common metabolic and degenerative diseases, cancer, and aging. The maternally inherited mtDNA contains 37 critical bioenergetic genes that are present in hundreds of copies per cell, but the 'mitochondrial genome' encompasses an additional 1,000-2,000 nuclear DNA (nDNA) mitochondrial genes. The interaction between these two mitochondrial genetic systems provides explanations for phenomena such as the non-Mendelian transmission of the common 'complex' diseases, age-related disease risk and progression, variable penetrance and expressivity, and gene-environment interactions. Thus, mtDNA genetics contributes to the quantitative and environmental components of human genetics that cannot be explained by Mendelian genetics. Because mtDNA is maternally inherited and cytoplasmic, it has fostered the first germline gene therapy, nuclear transplantation. However, effective interventions are still lacking for existing patients with mitochondrial dysfunction.

Therapeutic regimen of L-arginine for MELAS: 9-year, prospective, multicenter, clinical research

Objective

To examine the efficacy and safety of the therapeutic regimen using oral and intravenous l-arginine for pediatric and adult patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS).

Methods

In the presence and absence of an ictus of stroke-like episodes within 6 h prior to efficacy assessment, we correspondingly conducted the systematic administration of oral and intravenous l-arginine to 15 and 10 patients with MELAS in two, 2-year, prospective, multicenter clinical trials at 10 medical institutions in Japan. Subsequently, patients were followed up for 7 years. The primary endpoint in the clinical trial of oral l-arginine was the MELAS scale, while that for intravenous l-arginine was the improvement rates of headache and nausea/vomiting at 2 h after completion of the initial intravenous administration. The relationships between the ictuses of stroke-like episodes and plasma arginine concentrations were examined.

Results

Oral l-arginine extended the interictal phase (p = 0.0625) and decreased the incidence and severity of ictuses. Intravenous l-arginine improved the rates of four major symptoms—headache, nausea/vomiting, impaired consciousness, and visual disturbance. The maximal plasma arginine concentration was 167 μmol/L when an ictus developed. Neither death nor bedriddenness occurred during the 2-year clinical trials, and the latter did not develop during the 7-year follow-up despite the progressively neurodegenerative and eventually life-threatening nature of MELAS. No treatment-related adverse events occurred, and the formulations of l-arginine were well tolerated.

Conclusions

The systematic administration of oral and intravenous l-arginine may be therapeutically beneficial and clinically useful for patients with MELAS.

Mitochondrial energy generation disorders: genes, mechanisms, and clues to pathology

Inherited disorders of oxidative phosphorylation cause the clinically and genetically heterogeneous diseases known as mitochondrial energy generation disorders, or mitochondrial diseases. Over the last three decades, mutations causing these disorders have been identified in almost 290 genes, but many patients still remain without a molecular diagnosis. Moreover, while our knowledge of the genetic causes is continually expanding, our understanding into how these defects lead to cellular dysfunction and organ pathology is still incomplete. Here, we review recent developments in disease gene discovery, functional characterization, and shared pathogenic parameters influencing disease pathology that offer promising avenues toward the development of effective therapies.

Defective mitochondrial rRNA methyltransferase MRM2 causes MELAS-like clinical syndrome

Defects in nuclear-encoded proteins of the mitochondrial translation machinery cause early-onset and tissue-specific deficiency of one or more OXPHOS complexes. Here, we report a 7-year-old Italian boy with childhood-onset rapidly progressive encephalomyopathy and stroke-like episodes. Multiple OXPHOS defects and decreased mtDNA copy number (40%) were detected in muscle homogenate. Clinical features combined with low level of plasma citrulline were highly suggestive of mitochondrial encephalopathy, lactic acidosis and stroke-like episodes (MELAS) syndrome, however, the common m.3243 A > G mutation was excluded. Targeted exome sequencing of genes encoding the mitochondrial proteome identified a damaging mutation, c.567 G > A, affecting a highly conserved amino acid residue (p.Gly189Arg) of the MRM2 protein. MRM2 has never before been linked to a human disease and encodes an enzyme responsible for 2'-O-methyl modification at position U1369 in the human mitochondrial 16S rRNA. We generated a knockout yeast model for the orthologous gene that showed a defect in respiration and the reduction of the 2'-O-methyl modification at the equivalent position (U2791) in the yeast mitochondrial 21S rRNA. Complementation with the mrm2 allele carrying the equivalent yeast mutation failed to rescue the respiratory phenotype, which was instead completely rescued by expressing the wild-type allele. Our findings establish that defective MRM2 causes a MELAS-like phenotype, and suggests the genetic screening of the MRM2 gene in patients with a m.3243 A > G negative MELAS-like presentation.

The Newcastle Pediatric Mitochondrial Disease Scale: translation and cultural adaptation for use in Brazil

ABSTRACT Objective The aim of this study was to translate and adapt the Newcastle Paediatric Mitochondrial Disease Scale (NPMDS) to Portuguese for use in Brazil. Methods The scale was applied in 20 pediatric patients with mitochondrial disease, in three groups: myopathy (n = 4); Leigh syndrome (n = 8); and encephalomyopathy (n = 8). Scores were obtained for the various dimensions of the NPMDS, and comparisons were drawn between the groups. Results There was a statistically significant difference between the myopathy group and the Leigh syndrome group (p = 0.0085), as well as between the myopathy and encephalomyopathy groups (p = 0.01). Conclusions The translation of the NPMDS, and its adaptation to the socioeconomic and cultural conditions in Brazil, make the NPMDS score useful as an additional parameter in the evaluation and monitoring of pediatric patients with MD in Brazil.

Bioenergetic medicine

Here we discuss a specific therapeutic strategy we call 'bioenergetic medicine'. Bioenergetic medicine refers to the manipulation of bioenergetic fluxes to positively affect health. Bioenergetic medicine approaches rely heavily on the law of mass action, and impact systems that monitor and respond to the manipulated flux. Since classically defined energy metabolism pathways intersect and intertwine, targeting one flux also tends to change other fluxes, which complicates treatment design. Such indirect effects, fortunately, are to some extent predictable, and from a therapeutic perspective may also be desirable. Bioenergetic medicine-based interventions already exist for some diseases, and because bioenergetic medicine interventions are presently feasible, new approaches to treat certain conditions, including some neurodegenerative conditions and cancers, are beginning to transition from the laboratory to the clinic.

Development of pharmacological strategies for mitochondrial disorders

Mitochondrial diseases are an unusually genetically and phenotypically heterogeneous group of disorders, which are extremely challenging to treat. Currently, apart from supportive therapy, there are no effective treatments for the vast majority of mitochondrial diseases. Huge scientific effort, however, is being put into understanding the mechanisms underlying mitochondrial disease pathology and developing potential treatments. To date, a variety of treatments have been evaluated by randomized clinical trials, but unfortunately, none of these has delivered breakthrough results. Increased understanding of mitochondrial pathways and the development of many animal models, some of which are accurate phenocopies of human diseases, are facilitating the discovery and evaluation of novel prospective treatments. Targeting reactive oxygen species has been a treatment of interest for many years; however, only in recent years has it been possible to direct antioxidant delivery specifically into the mitochondria. Increasing mitochondrial biogenesis, whether by pharmacological approaches, dietary manipulation or exercise therapy, is also currently an active area of research. Modulating mitochondrial dynamics and mitophagy and the mitochondrial membrane lipid milieu have also emerged as possible treatment strategies. Recent technological advances in gene therapy, including allotopic and transkingdom gene expression and mitochondrially targeted transcription activator-like nucleases, have led to promising results in cell and animal models of mitochondrial diseases, but most of these techniques are still far from clinical application.

Selected case from the Arkadi M. Rywlin International Pathology Slide Series: Mitochondrial myopathy presenting with chronic progressive external ophthalmoplegia (CPEO): a case report

A 43-year-old female patient diagnosed with chronic progressive external ophthalmoplegia (CPEO) because of mitochondrial myopathy documented by muscle biopsy is presented. The chief complaints were represented by blepharoptosis and ophthalmoplegia. The muscle biopsy was evaluated by histology, using the appropriate histochemical and histoenzimological stains. Ragged red fibers with Gomori trichrome stain were seen, which showed cytochrome c oxydase deficiency and abnormal succinate dehydrogenase staining in around 20% of muscle fibres. Electron microscopy was also performed which demonstrated abnormal, hyperplastic, pleomorphic, and hypertrophic mitochondria, characterized by paracrystalline inclusions arranged in parallel rows ("parking-lot" inclusions), consisting of rectangular arrays of mitochondrial membranes in a linear or grid-like pattern. In conclusion, mitochondrial myopathy was definitely diagnosed. Although molecular analysis, which was subsequently carried out, failed to reveal mutations in the mitochondrial DNA or in selected nuclear genes, the pathologic diagnosis was not changed. The differential diagnosis of CPEO with other forms of ocular myopathies as well as the possible association of CPEO with systemic syndromes is discussed. Ophtalmologists and medical internists should always suspect CPEO when dealing with patients affected by ocular myopathy, either in its pure form or in association with other myopathic or systemic signs.

Mitochondrial disease heterogeneity: a prognostic challenge

Mitochondrial diseases are a heterogeneous group of progressive, genetically transmitted, multisystem disorders caused by impaired mitochondrial function. The disease course for individuals with mitochondrial myopathies varies greatly from patient to patient because disease progression largely depends on the type of disease and on the degree of involvement of various organs which makes the prognosis unpredictable both within the same family and among families with the same mutation. This is particularly, but not exclusively, true for mitochondrial disorders caused by mtDNA point mutations, which are maternally inherited and subject to the randomness of the heteroplasmy. For this reason, the prognosis cannot be given by single mitochondrial disease, but should be formulated by any single mitochondrial disease-related event or complication keeping in mind that early recognition and treatment of symptoms are crucial for the prognosis. The following approach can help prevent severe organ dysfunctions or at least allow early diagnosis and treatment of disease-related complications.